Oxalate coating on nonferrous metal



Patented Nov. 10, 1936 Leo P. Gnrtin, Cranbnry, N. J., assignor toCurtin- Howe Corporation, New York, N. Y., a corporation of New York NoDrawing. Application January 31, 1933. Serial No. 654,551

3 Claims. (cl. 148-6) In an application Serial No. 591,932, filedFebruary 9, 1932, issued January 31, 1933, as U. 8. Patent No.1,895,569, in the name of Leo P. Curtin and Bernard L. Kline, there isdescribed a 5 method of producing protective coatings on iron and steelarticles by subjecting them to the action of a bath in which ferricoxalate is the principal component.

Most of the non-ferrous metals below calcium in the electromotive seriesacquire coatings as a result of oxidation by air, these coatings in someinstances, because of their impervious nature, providing the underlyingmetal with partial or complete protection against further oxidation. Itis often the case, however, that such coatings fail to give the expectedprotection and oxidation of the metal continues at a rapid rate. This isespecially true in the case of zinc and some of the new alloys whichcontain magnesium.

Furthermore, it is sometimes desired to apply a lacquer, enamel or paintcoating over such metals, and if oxidation takes place after theapplication of such a coating, the enamel or lacquer film is disrupted.-In addition, the physical nature of such metallic surfaces is often suchthat the lacquer or other coating material does not make a satisfactorybond with the metal. This is particularly the case with zinc, whether itbe zinc sheets or castings or the thin coating of metallic zinc overiron produced by the galvanizing process.

Now I have discovered that similar coatings may be formed in a similarmanner on nonferrous metals. The invention will be described andillustrated hereinafter by reference to the coating of zinc, it beingunderstood that the invention is not limited thereto but is applicablegenerally for coating metals which react with ferric oxalate to theformation of ferrous oxalate and the oxalate of the metal.

The surface of freshly galvanized material or of newly made zinccastings is particularly diiilcult to paint or lacquer. The coatingliquid appears to make a good bond at the time of its application, butsome time later it frequently peels off in large flakes showing theunderlying zinc metal as bright and clean as though the paint or lacquerhad never been applied. Furthermore, solid zinc objects and galvanizedmaterials are subject to a form of oxidation known as white rust whichseems to take place as well when such articles are held in storage aswhen exposed to the elements. This white rust, which 55 is supposed toconsist of oxides and carbonates of zinc, is the cause of manyrejections by purchasers. I

I have found that, if zinc is immersed ina hot aqueous solution offerric oxalate containing barely enough free oxalic acid to inhibithydroly- 5 sis of the ferric salt, the zinc receives alight butcontinuous coating which very slightly roughens the surface and providesan admirable base for a subsequent coating of enamel, lacquer, paint oroil. When zinc is so treated the lacquer or other coating clingstenaciously and shows no disposition to peel oil as is the case when itis applied directly over the bright metal.

The coating consists of a complexmixture of oxalates of iron and zinc.The following chemi- 15 cal reaction illustrates how the coating may beproduced on the surface of the metal, although other reactions mayoccur.

It will be noticed that the coating consists of a mixture of insolubleoxalates of zinc and iron and that according to the foregoing equationthe zinc metal provides only one-third of the metal going into thecoating, the remaining two-thirds 20 being obtained from the ironoxalate in solution. This is an important advantage particularly in thecase of galvanized materials. It is known that it is possible to producea coating on metallic zinc by means of oxalic acid alone. This coatingis not nearly so adherent or otherwise satisfactory, however, and itconsumes three times as much of the zinc metal in its formation v aswhen ferric oxalate is used. The coating formed by oxalic acid is quitelikely to be nonadherent and in such cases can be removed from the metalby brushing or wiping. This difliculty has not been encountered with theferric oxalate type of bath. The temperature of the bath is not at allcritical, good results being obtained at temperatures from 50 C. to 99C. The concentration of ferric oxalate may vary within considerablelimits, say from 1% to 10%. I prefer to use, however, a bath containing4% to 5% 40 of ferric oxalate and. 0.5% to 1.0% of free oxalic acid.

Among other non-ferrous metals capable of being coated by this processmay be mentioned aluminum, magnesium and 'lead.

I claim:

1. The process of coating 9. non-ferrous mtal of the class consisting ofzinc, aluminum, magnesium and lead, which'comprises exposing a surfaceof the metal to the action of an aqueous 2 2,oso,ses

solution containing from i to 10 per cent of ferric oxalate. until acoating of the desired thickness is formed thereon.

2. A process as set forth in claim 1, wherein the solution of ferricoxalate is acidulated merely to such extent as to prevent hydrolysis oithe ferric oxalate.

3. The process of coating 9. non-ferrous metal 02 the class consistingof zinc, aluminum, magnesium and lead, which comprises exposing asurfaceof the metal to the action of an aqueous solution containing from 4 to 5per cent of ferric oxalate and from 0.5 to 1.0 per cent of oxalic acid,until a coating 0! the desired thickness is formed thereon.

LEO P. CURTm.

